Fluidized bed pellet reactor water softener and process for softening water

ABSTRACT

As pellets grow from seed/sand in a fluidized bed pellent reactor, the weight of the reactor is measured and the density of the contents of the reactor is calculated, and the input flow of untreated water, water treatement chemical, and seed/sand are adjusted to provide improved removal of water hardness while reducing fine particulates in the outflow of softened water from the reactor.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to fluidized bed pellet reactors.

2. Brief Description of the Prior Art

Fluidized bed pellet reactors have been employed to soften hard water ona municipal scale in the Netherlands for over 40 years (also known as“Amsterdam” reactors), and approximately half the water supply there iscurrently softened using such reactors. In these reactors a fine sand(“garnets”) is suspended in an upward flowing column of water to betreated, and a reagent (such as sodium hydroxide, or sodium carbonate)is added to supersaturate with calcium carbonate (CaCO₃). The calciumcarbonate precipitates or crystallizes on the fine sand “seeds” formingpellets composed largely of solid calcium carbonate, which areperiodically removed from the reactor. Treated water is removed from thetop of the fluid column. The precipitation process is controlled byadjusting the flows of seed, input water, and reagent. The effects ofpellet size, pH, height of the fluid bed, influent flow, and refluxratio (ratio of the part of the effluent flow refluxed to influentflow), and influent flow on hardness removal have been studied.Typically, pellets reach about 1 mm to 2 mm and are discharged.Crystallization takes place largely in the bottom layer of the fluidizedbed. Despite long experience and extensive study, Ca⁺⁺ and totalhardness removal is not ensured in current reactors. Alternatives havebeen proposed to improve treatment, such as by circulating the pelletsfrom the top to the bottom of the fluidized bed.

There are multiple problems with current fluidized bed pellet reactorsemployed as water softeners. For example, compaction of the fluidizedbed leads to dead zones with resultant short-circuiting. Sometimes suchcompaction occurs to the point of settling enough seed product that thenozzles are covered and essentially “plugged”. Further, the requiredhigh speed of the pump required to maintain the fluidized bed leads topremature wear on the pump and abrasion of the crystals with the obviousresultant increase in fines production. Another problem is theproduction of excessive fines in the effluent overflow which leads todownstream filter limitations. Specifications for the effluent typicallyhave limits on effluent turbidity. Yet another issue is that theinfluent water feed rates vary which affects the fluidization of thebed. Current control schemes rely on pumps and timers to feed seed/sandinto the Amsterdam fluidized bed reactor and also to “blown-down” thefinished product. All the data is volumetric and the accuracy and valueof the data would increase by adding weight to the formulas.

SUMMARY OF THE INVENTION

The present invention provides a method of growing crystals in afluidized bed reactor that utilizes a weight based operating systemand/or in conjunction with a volumetric operating system to achieve abed of a known density. As the process continues, the weight of the bedwill be indicated on a scale which is used to determine that rate of thecrystal precipitation, knowing this rate improves operational efficiencyand overall water quality. Previously the batches were fedvolumetrically “blown-down” after a predetermined period of time.

In one aspect of the present invention, a seed/sand feed sub-system isprovided. In this sub-system, a predetermined weight of seed/sand is fedinto a predetermined weight or volume of water.

In another aspect, a pellet reactor is initially filled with water todetermine a known weight without seed/sand. The pellet reactor ismounted on load cells such that the total weight can be monitored. Whenthe seed/sand is loaded, the measured weight of the pellet reactor willincrease. As the precipitative crystallization process proceeds, theweight of the pellet reactor will continue to increase. From themeasured changes in weight the rate of increase can be calculated andsubsequently optimize the efficiency of the reaction. As the bed builds,the density of the bed as well as any changes in density can bedetermined. A predetermined density is targeted for optimum benefits.

In another aspect of the present invention, the speed of the water flowpump can be adjusted to adjust the density of the bed alone oroptionally in conjunction with an upper baffle. This will affect changesin the rise rate while at the same time reduce the speed of the pump.Wear is a function of square of the pump speed as is the rate ofproduction of fines. An optimum rise rate is maintained to minimize thecarry-over of fines into the effluent.

When the density of the bed reaches optimum condition, some of theseed/sand crystals is removed or blown-down to a storage hopper. Theblown-down material is a combination of crystals and water. The water ispreferably removed and drained away from the hopper. Preferably, thehopper is mounted on load cells to indicate how much weight is in thehopper. The subsequent loss in weight reflects the progress of thegravity dewatering of the pellets. Collectively the hoppers will alsoprovide useful inventory management information and truck loadingcontrol options.

In one aspect the present invention provides a fluidized bed pelletreactor for softening hard water. In this aspect the reactor comprises areactor tank for containing a fluidized bed of pellets; means forproviding fine sand for use as crystallization seeds to the reactortank; an inflow pump for pumping inflow fluid to be treated into thereactor tank to fluidize the bed of pellets; means for providing reagentto the reactor tank for adjusting the pH of the fluid in the reactortank; means for withdrawing treated water as an effluent from thereactor tank; means for withdrawing pellets from the reactor tank; andmeans for measuring the weight of the reactor tank and the contents ofthe reactor tank.

In this aspect, the fluidized bed pellet reactor preferably furtherincludes a baffle positioned at the top of the reactor tank for limitingthe outflow of effluent from the reactor tank. Further, preferably themeans for providing fine sand includes means for measuring the weight ofthe fine sand provided, for example, one or more load cells. Inaddition, preferably the means for providing reagent includes means formeasuring the weight of the reagent provided, such as one or more loadcells. Further, preferably the fluidized bed pellet reactor includesmeans for transporting pellets withdrawn from the reactor tank, such asa conveyor, to a means for storing the pellets, such as one or morestorage silos. Preferably, the fluidized bed pellet reactor furtherincludes means for adjusting the height of the fluidized bed, such as byvarying the speed of the inflow pump. Preferably, the means formeasuring the weight of the reactor tank includes a plurality of loadcells for sensing the distribution of weight within the reactor tank.Preferably, in this aspect the fluidized bed pellet reactor furtherincludes means for controlling the speed of the inflow pump. Preferably,in this aspect the fluidized bed pellet reactor further includes meansfor measuring the height of the fluidized bed within the reactor tank,such as an acoustic sensing device. Preferably, in this aspect the meansfor providing fine sand includes a sand storage tank and means fordelivering sand from the sand storage tank to the reactor tank, such asa feed pump. Preferably, in this aspect the means for providing reagentto the reactor tank includes a reagent storage tank and means fordelivering reagent from the reagent storage tank to the reactor tank,such as a feed or metering pump.

In another aspect, the present invention provides a fluidized bed pelletreactor for softening hard water. In this aspect, the reactor comprisesa seed storage tank for storing fine sand for use as crystallizationseeds, a reactor tank for containing a fluidized bed of pellets, aninflow pump for pumping inflow fluid to be treated into the reactor tankto fluidize the bed of pellets, a reagent storage tank for storing achemical reagent for adjusting the pH of the fluid in the reactor tank,means for delivering seed from the seed storage tank to the reactortank, means for delivering reagent from the reagent storage tank to thereactor tank, means for withdrawing treated water as an effluent fromthe reactor tank, means for withdrawing pellets from the reactor tank,and means for measuring the weight of the reactor tank and the contentsof the reactor tank. In one presently preferred embodiment, thefluidized bed pellet reactor further includes a baffle positioned at thetop of the reactor tank for limiting the outflow of effluent from thereactor tank. Preferably, the fluidized bed pellet reactor of thepresent invention further includes means for measuring the weight of theseed storage tank and the contents of the seed storage tank. Preferably,the fluidized bed pellet reactor of the present invention furtherincludes means for adjusting the height of the fluidized bed.Preferably, the fluidized bed pellet reactor of the present inventionfurther includes means for measuring the weight of the reagent tank andthe contents of the reagent tank. Preferably, in another aspect, themeans for measuring the weight of the reactor tank includes a pluralityof load cells for sensing the distribution of weight within the reactortank. Preferably, the fluidized bed pellet reactor of the presentinvention further includes means for controlling the speed of the inflowpump. Preferably, the fluidized bed pellet reactor of the presentinvention further includes means for measuring the height of thefluidized bed within the reactor tank.

The present invention also provides a process for softening water. Theprocess includes providing a fluidized bed pellet reactor for softeninghard water. The reactor preferably includes a seed storage tank forstoring fine sand for use as crystallization seeds, a reactor tank forcontaining a fluidized bed of pellets, an inflow pump for pumping inflowfluid to be treated into the reactor tank to fluidize the bed ofpellets, a reagent storage tank for storing a chemical reagent foradjusting the pH of the fluid in the reactor tank, means for deliveringseed from the seed storage tank to the reactor tank, means fordelivering reagent from the reagent storage tank to the reactor tank,means for withdrawing pellets from the reactor tank, and means formeasuring the weight of the reactor tank and the contents of the reactortank. The process further includes mixing a predetermined weight of seedwith a predetermined volume or weight of water in the seed storage tankto form a mixture of seed and water; filling the reactor tank with apredetermined volume of water; transferring a predetermined amount ofthe mixture of seed and water from the seed storage tank to the reactortank; pumping inflow liquid including calcium ion to be treated into thereactor tank to create a fluidized bed; delivering reagent from thereagent storage tank to the reactor tank to crystallize the calcium ionas calcium carbonate on the seed to form pellets; maintaining thedensity of the fluidized bed at a predetermined optimum density;withdrawing treated water as an effluent from the reactor tank;withdrawing pellets from the reactor tank. Preferably, the processincludes controlling the speed of the inflow pump to maintain thepredetermined density of the fluidized bed. Preferably, the processfurther includes providing means for measuring the weight of the seedstorage tank and the contents of the seed storage tank. Preferably, theprocess further comprises providing a hopper for receiving pellets fromthe reactor, the hopper including means for measuring the weight of thehopper, permitting gravity dewatering of the pellets received from thereactor, and monitoring the loss of weight of the pellets as the pelletsare dewatered. Preferably, the process wherein the means for measuringthe weight of the reactor tank includes a plurality of load cells forsensing the distribution of weight within the reactor tank, the processfurther comprises sensing the distribution of weight of the contents ofthe reactor tank and comparing the sensed distribution of weight with apredetermined distribution. Preferably, the process further comprisesdelivering seed/sand to the seed storage tank; determining the weight ofthe seed/sand in the storage tank; delivering water to the seed storagetank; and measuring the weight of the water in the storage tank, whereinthe water and sand are delivered in quantities sufficient to achieve apredetermined ratio of sand to water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a fluidized bed pellet reactorof the prior art.

FIG. 2 is a schematic representation of a fluidized bed pellet reactorof the present invention.

FIG. 3 is a fragmentary schematic bottom plan view of a reactor tankaccording to one presently preferred aspect of the present invention.

FIG. 4 is a schematic elevational view of a sand cleaning subsystem forpreparing seed for use in the present invention.

DETAILED DESCRIPTION

Referring now to the figures in which like reference numerals representlike elements in each of the several views, there is shown in FIG. 1, aschematic representation of a fluidized bed pellet reactor 10 of theprior art, commonly referred to as an “Amsterdam” reactor. An influxflow 12 of water to be treated is pumped into the bottom of a generallyvertical reactor tank 14. The water flows upward in the tank 14 in thedirection shown by the arrows. At the same time, a basic reagent 16 suchas sodium hydroxide or sodium carbonate is pumped into the bottom of thetank 14 and dispersed in the upward flow of the influx water. Seed orfine sand 22 is admitted into the tank 14 and is suspended in the tank14 by the upward flow of the water in the tank 14, forming a fluidizedbed. Calcium ions dissolved in the upflowing, basic, water precipitateor crystallize on the seeds, thus removing dissolved calcium to “soften”the water and forming pellets The softened water, or treated effluent24, is discharged from the upper portion of the tank 14. Periodically,the largest pellets, which tend to settle to the bottom of the tank 14,are discharged 18 from the reactor 10 and recycled for industrial oragricultural purposes.

FIG. 2 is a schematic representation of fluidized bed pellet reactor 100of the present invention for carrying out the process of the presentinvention. The pellet reactor 100 includes a generally vertical reactortank 102 for containing the fluidized bed 116. The flow of influentwater 112 to be treated is provided near the bottom of the tank 102 byan influent pump 132 from calcium ion containing water 130 deliveredfrom an external source. A chemical reagent or reactant such as sodiumhydroxide or sodium carbonate is stored in a reagent storage tank 140and delivered to the reactor tank 102 near the bottom thereof bysuitable means such as by reagent metering or dosing pump 142. Seed orfine sand mixed with water is stored in a seed storage tank 150 and themixture is delivered 120 to the reactor tank 102 at a predeterminedelevation above the bottom of the reactor tank 102. Seed inflow 120 iscontrolled by suitable means 152, such as by a valve, a metering ordosing pump, or by some combination thereof. As the seed enters thebasic upward flow of the calcium ion containing water to be treated,calcium carbonate precipitates or crystallizes on the surface of theseed, forming pellets composed largely of calcium carbonate. The pelletsso formed are periodically discharged 118 from the bottom of the reactortank 102 and stored in a pellet hopper 160 where water 250 is permittedto drain from the wet pellets. Pellet removal is controlled by suitablemeans 162, such as by a valve or pump, or a combination of the two.Treated effluent 122 is discharged from the upper end of the reactortank 102 through an outflow pipe, outflow valve, or other suitablemeans. A baffle 124 is provided at or near the upper end or top of thereactor tank to limit the surface area of the top of the reactor tank102 so as to limit the rise rate of the fluidized bed 116 in the reactortank 102. Preferably, a device 260 for sensing the height of thefluidized bed 116 in the reactor tank 102 is provided, such as anacoustic, magnetostrictive or like device.

Operation of the pellet reactor 100 is preferably controlled by acentral controller, which can be a specialized controller or a generalpurpose computer 200. The reactor tank 102 is provided by one or moreload cells 210 which are connected to the computer 200 by one or moresuitable sensing lines 222, such that the weight of the reactor tank 102can be monitored continuously as the reactor tank 102 is filled withinfluent 112, reagent 114, and seed 120, and as pellets are formed onthe seed, thus permitting the weight/or and density of the fluidized bed116 within the reactor to be monitored. Preferably, the turbidity and/ortotal hardness of the effluent are also monitored (not shown), so thatthe input parameters, the input pump speed, rate of reagent addition,and rate and quantity of seed addition can be adjusted to provide adesired level of hardness reduction while minimizing release ofparticulate fines in the effluent which must be subsequently removed,such as by filtration.

The height of the fluidized bed 116 in the reactor tank 102 can bemonitored and measured by a scale of suitable means, for example,acoustically. The average density of the fluidized bed 116 can becalculated based upon weight and volume.

The inflow pump 132 can be controlled by the computer 200 through asuitable control line. The speed of the inflow pump 132 can be variedand/or a valve can be used to adjust the pumping rate (not shown).

The computer 200 can optionally monitor the weight of the seed storagetank 150 by suitable means such as one or more load cells 214 through asuitable sensor line 218. Thus, seed storage tank 150 can be tared, andas water and seed/sand are sequentially added to the seed storage tank150, the weight ratio of the two can be calculated, and the amount ofseed/sand/water 120 delivered to the reactor tank 102 can be monitored.

The computer 200 can optionally also monitor the weight of the reagentstorage tank 140 (and thus the weight of the reagent) by suitable meanssuch as by one or more load cells 212 and corresponding sensing line224, and control operation of the means for delivering reagent 142 (i.e.the valve or metering pump) to the reactor tank 102 through a suitablecontrol line 226. Thus, the rate at which reactant 114 is delivered tothe reactor tank 102 can be optimized to maximize the removal ofdissolved calcium from the influent water 112.

Examples of reagents that can be employed in the process of the presentinvention include Ca(OH)₂ slurry; quicklime, hydrated lime, sodiumhydroxide, soda ash, calcium sulfate, lime-soda ash, and caustic soda.In addition, sundry additives can be includes, and the seed bed or aportion of the seed bed can be recirculated.

In addition, the weight of the pellet hopper 160 is sensed by one ormore load cells 216 and then provided to the computer 200 through sensorline 228. After wet pellets have been discharged from the reactor tank102, drying of the pellets can be monitored as the sensed weight of thepellets declines as water 250 drains from the pellet hopper 160, and thedried pellets 252 can be thus discharged from the pellet hopper 160 whena predetermined level of dryness has been obtained. Discharge of pelletsfrom the reactor tank 102 can be controlled by any suitable means, suchas by a valve 162. Operation of the valve 162 can be controlled by thecomputer 200 through a control line 232. The dried pellets can betransferred, for example, by a conveyor, to storage bins, for latertransfer to trucks, for example. The pellets can be processed, stored instorage bins, and discarched into trucks for remote transport, such asdisclosed, for example, in U.S. Pat. Nos. 6,698,766, and 8,682,477, theentire disclosures of which are incorporated herein by reference.

In another presently preferred embodiment of the present invention, aplurality of load cells 310, 312, 314, 316 (FIG. 3) are provided underthe supporting legs of the reactor tank 306, so that the distribution ofthe weight of the fluidized bed in the reactor tank 306 can bemonitored, and a distribution deviating from uniform can be corrected.

Seed sand for use in the process of the present invention can beprepared using a suitable subsystem 400, such as illustrated in FIG. 4.In the process and apparatus of the subsystem 400, sand which may havevarious contaminants, is transferred from a suitable store (not shown)by a conveyor 402 equipped with load cells 404 for measuring the weightof sand being transported by the conveyor 402, and delivered to a hopper406, which is supported by a plurality of legs 408, and which are alsoprovided with load cells 410 for monitoring the weight of sand deliveredto be washed. Sand is discharged through a flow-control valve 414 to anagitator or agitation tank 430, which is supplied proximate the bottomof the agitation tank 430 with clean wash water 434 through a modulatingvalve 438 for controlling the rate of flow of the clean water into thetank. The rate of flow of clean water into the agitation tank 430 ismonitored with a flow meter 436. As the clean water flows upwardsthrough sand in the agitation tank 430, impurities are removed, and theresulting dirty water is removed through an outlet 420 at the top of theagitation tank 430. As need be, clean water 412 can be supplied to theagitation tank 430 near the top of the agitation tank 430 to clean theagitation tank 430, and the resulting dirty water can be dischargedthrough a tank drain 450 at the bottom of the tank. Clean seed sand 440can be discharged through a valve near the bottom of the agitation tank430.

Various modifications can be made in the details of the variousembodiments of the apparatus and process of the present invention, allwithin the scope and spirit of the invention and defined by the appendedclaims.

The invention claimed is:
 1. A fluidized bed pellet reactor forsoftening hard water, the reactor comprising: a reactor tank forcontaining a fluidized bed of pellets; means for providing fine sand foruse as crystallization seeds to the reactor tank; an inflow pump forpumping inflow fluid to be treated into the reactor tank to fluidize thebed of pellets; means for providing reagent to the reactor tank foradjusting the pH of the fluid in the reactor tank; means for withdrawingtreated water as an effluent from the reactor tank; means forwithdrawing pellets from the reactor tank; and means for measuring theweight of the reactor tank and the contents of the reactor tank, whereinthe means for measuring the weight of the reactor tank includes aplurality of load cells for sensing the distribution of weight withinthe reactor tank.
 2. The fluidized bed pellet reactor according to claim1 further including a baffle positioned at the top of the reactor tankfor limiting the outflow of effluent from the reactor tank.
 3. Thefluidized bed pellet reactor according to claim 1 wherein the means forproviding fine sand includes means for measuring the weight of the finesand provided.
 4. The fluidized bed pellet reactor according to claim 1wherein the means for providing reagent includes means for measuring theweight of the reagent provided.
 5. The fluidized bed pellet reactoraccording to claim 1 further comprising means for transporting pelletswithdrawn from the reactor tank to a means for storing the pellets. 6.The fluidized bed pellet reactor according to claim 1 further includingmeans for adjusting the height of the fluidized bed.
 7. The fluidizedbed pellet reactor according to claim 1 further including means forcontrolling the speed of the inflow pump.
 8. The fluidized bed pelletreactor according to claim 1 further including means for measuring theheight of the fluidized bed within the reactor tank.
 9. The fluidizedbed pellet reactor according to claim 1 wherein the means for providingfine sand includes a sand storage tank and means for delivering sandfrom the sand storage tank to the reactor tank.
 10. The fluidized bedpellet reactor according to claim 1 wherein the means for providingreagent to the reactor tank includes a reagent storage tank and meansfor delivering reagent from the reagent storage tank to the reactortank.